(Non-)Thermal Production of WIMPs during Kination

Abstract

Understanding the nature of the Dark Matter (DM) is one of the current challenges in modern astrophysics and cosmology. Knowing the properties of the DM particle would shed light on physics beyond the Standard Model and even provide us with details of the early Universe. In fact, the detection of such a relic would bring us information from the pre-Big Bang Nucleosynthesis (BBN) period, an epoch from which we have no data, and could even hint at inflationary physics. In this work, we assume that the expansion rate of the Universe after inflationary is governed by the kinetic energy of a scalar field ϕ, in the so-called “kination” model. We assume that the ϕ field decays into both radiation and DM particles, which we take to be Weakly Interacting Massive Particles (WIMPs). The present abundance of WIMPs is then fixed during the kination period through either a thermal “freeze-out” or “freeze-in” mechanism, or through a non-thermal process governed by the decay of ϕ. We explore the parameter space of this theory with the requirement that the present WIMP abundance provides the correct DM relic budget. Requiring that BBN occurs during the standard cosmological scenario sets a limit on the temperature at which the kination period ends. Using this limit and assuming the WIMP has a mass mχ = 100 GeV, we obtain that the thermally-averaged WIMP annihilation cross section has to satisfy the constraints 3.5 × 10−16 GeV−2 ≲ (σv) ≲ 1.4 × 10−5 GeV−2 in order for having at least one of the production mechanism to yield the observed amount of DM. This result shows how the properties of the WIMP particle, if ever measured, can yield information on the pre-BBN content of the Universe.

Growth of dark matter perturbations during kination

“…We also found a fitting function that not only smoothly connects Eqs. (26) and (27), but also matches the numeric solution of δ χ to within 5% for modes with 0.05 < k/k KR < 1000: (27), it is clear that if a mode enters the horizon during an era of kination, δ χ /Φ 0 ∝ k 1/2 . In contrast, it has been shown in Refs.…”

“…With this scaling, we can determine how the matter power spectrum scales with k for modes with 4), while the long-dashed line and shortdashed line represent the analytical evolution via Eqs. ( 26) and (27).…”

“…[18], we explored how the dark matter density evolves if it is thermally produced during an era of kination, and we derived analytic expressions for the dark matter relic abundance; see also Refs. [19][20][21][22][23][24][25][26][27][28]. To obtain the observed dark matter relic abundance, dark matter that is thermally produced during an era of kination requires larger annihilation cross sections than dark matter that is thermally produced during radiation domination.…”

“…(4), while the long-dashed line and shortdashed line represent the analytical evolution via Eqs (26). and(27).…”

Dark Matter Freeze-in Production in Fast-Expanding Universes

“…We note that freeze-in through pair production of DM particles (case 3 above) but limited to the specific case n = 2 (kination domination) was studied in Refs. [24,25]. The goal of this paper is to present, instead, a general analysis for different cosmological histories and freeze-in scenarios.…”